Abstract

We demonstrate numerically that metal-insulator-metal (MIM) configurations in which the top metal layer consists of a periodic arrangement of nanobricks, thus facilitating gap-surface plasmon resonances, can be designed to function as efficient and broadband quarter-wave plates in reflection by a proper choice of geometrical parameters. Using gold as the metal, we demonstrate quarter-wave plate behavior at λ ≃ 800 nm with an operation bandwidth of 160 nm, conversion efficiency of 82%, and angle of linear polarization fixed throughout the entire bandwidth. This work also includes a detailed analytical and numerical study of the optical properties and underlying physics of structured MIM configurations.

Figures (5)

(a) Sketch of a three-medium system with a metasurface positioned at the interface between medium 1 and 2. The incident plane wave propagates normal to the interfaces in the z-direction with polarization along the major axis of the prolate spheroids. The reflection coefficient in Eq. (4) corresponds to ‘r’ in the drawing. (b) Amplitude and (c) phase of the reflection coefficient for three different cases: (I) No metasurface and medium 3 is assumed to be gold [equivalent to Z3 = ZAu and αii = 0 in Eq. (4)], (II) Structured MIM configuration with medium 3 as gold [equivalent to Z3 = ZAu in Eq. (4)], (III) Metasurface in homogeneous surroundings [equivalent to Z3 = ZAir in Eq. (4)]. The other parameters are: medium 1 and 2 are assumed to be air, the prolate gold spheroids have major and minor axes 71 nm and 12 nm, respectively, the period is Λ = 230 nm, thickness of medium 2 is ts = 150 nm, and the gold permittivity is described by interpolated experimental data [33].

(a) Resonance wavelength λres as a function of the strip width w for the continuous-layer GSP resonator depicted in the inset. In the calculations, the metal strip and substrate are assumed to be gold and the spacer is SiO2 with refractive index ns = 1.45. The height of the strip is h = 50 nm. (b) Sketch of the structured MIM unit cell used in the design of a quarter-wave plate in reflection. The metal parts are assumed to be gold, the spacer is SiO2, and the upper dielectric is air. In this work, the nanobrick dimensions are fixed at t = 50 nm, Lx = 138 nm, Ly = 105 nm, and the corners are rounded with a radius of 5 nm.